Nectarines.

I bought my brother a nectarine and a fig tree as a gift after staying with him in Berwickshire a few years ago. although he is well North, he has a wonderfull long greenhouse lean too against a stone wall facing south, with a concrete floor. He cut square holes in the concrete and planted them both, restricting the roots of the fig as instructed by lining the hole. He has had fantastic results with the fig giving up to 200 fruit for next year and masses this year beautiful. The nectarine is equally prolific in the fruit but they are very small not any bigger than a small plum. The leaves go yellowy and crinckly can anyone give a solution to the problem. The fruit are edible and sweet. It sounds to me like a deficiency and not enough water, but he said it gets ample regular watering and he puts plenty of their farm yard manure around them. He wondered if they were too far north, or was it the type. I have lost the receipt so sorry that I cannot give the name of it, but I am sure I would have been very selective when I ordered it from the Nursery. He had a nectarine when he lived in South Wales in the green house which was magnificent so he is quite used to growing them. Can any expert out there throw any light on the problem please. He does pick off a lot of the fruit through the growing season hopeing to enlarge the remaining fruit with no luck.